Prostate cancer is the second most prevalent cancer among men in the United States and ranks second to lung cancer in terms of annual mortality (Weir et al., 2003, Annual Report to the Nation on the Status of Cancer, 1975-2000, featuring the uses of surveillance data for cancer prevention and control, J. Natl. Cancer Inst., 95:1276-1299). Prostate cancer can begin as an androgen-dependent tumor that undergoes clinical regression in response to pharmacological and surgical strategies that reduce testosterone concentration can be effective in treating prostate cancer at this stage. However, despite this treatment, the cancer can eventually regrow as a lethal androgen or hormone-independent tumor (Feldman et al., 2001, The Development of Androgen-Independent Prostate Cancer, Nat. Rev. Cancer, 1:34-45).
The molecular basis for hormone-independent cancer progression is poorly understood. However, most androgen-independent prostate cancers still express androgen receptor (AR) and aberrant AR signaling is postulated to be an important mechanism of progression to androgen independence (Taplin et al., 2004, Androgen Receptor: A Key Molecule in the Progression of Prostate Cancer to Hormone Independence, J. Cell Biochem, 91:483-490). The pathways involved in AR-mediated survival of prostate cancer cells in the absence of androgens include amplification or mutation of the AR, deregulation of growth factors or cytokines, and alteration of AR coregulators. Thus, the AR is a key molecule in pathways leading to androgen independence.
Both increased sensitivity of AR to androgens and activation of AR by growth factor and/or cytokines are proposed to account for androgen-independent growth (Feldman et al., 2001, The Development of Androgen-Independent Prostate Cancer, Nat. Rev. Cancer, 1:34-45). Clinically, upregulation of the AR has been recently demonstrated to be consistently associated with hormone-independent disease (Edwards et al., 2003, Androgen Receptor Gene Amplification and Protein Expression in Hormone Refractory Prostate Cancer, Br. J. Cancer, 89:552-556) (Taplin et al., 1995, Mutation of the Androgen-Receptor Gene in Metastatic Androgen-Independent Prostate Cancer, N. Engl. J. Med., 332:1393-1398). In a xenograft model, Chen et al. (Chen et al., 2004, Molecular Determinants of Resistance to Antiandrogen Therapy, Nat. Med. 10:33-39) recently reported that a two- to five-fold increase in AR mRNA was the only change consistently associated with the development of androgen resistance. As a result, cells exhibit an increased sensitivity to low levels of androgen. In addition, androgen receptor antagonists are converted to agonists.
Thus, inhibition of AR expression is key to the design of new agents effective for treatment of prostate cancer (Isaacs et al., 2004, Androgen Receptor Outwits Prostate Cancer Drugs, Nat. Med., 10:26-27). Strategies that target the AR include the use of RNA interference, ribozymes, and antisense molecules to decrease AR mRNA (Eder et al., 2002, Inhibition of LNCaP Prostate Tumor Growth In Vivo by an Antisense Oligonucleotide Directed Against the Human Androgen Receptor, Cancer Gene Ther., 9:117-125) (Zegarra-Moro et al., 2002, Disruption of Androgen Receptor Function Inhibits Proliferation of Androgen-Refractory Prostate Cancer Cells, Cancer Res., 62:1008-1013), heat shock protein 90 inhibitors, such as 17 AAG, to destabilize AR protein (Solit et al., 2002, 17-Allylamino-17-Demethoxygeldanamycin Induces the Degradation of Androgen Receptor and HER-2/neu and Inhibits the Growth of Prostate Cancer Xenografts, Clin. Cancer Res., 8:986-993), and pharmacological inhibitors of AR protein synthesis or function (Mitchell et al., 1999, Resveratrol Inhibits the Expression and Function of the Androgen Receptor in LNCaP Prostate Cancer Cells, Cancer Res., 59:5892-5895) (Zhu et al., 2001, Silymarin Inhibits Function of the Androgen Receptor by Reducing Nuclear Localization of the Receptor in the Human Prostate Cancer Cell Line LNCaP, Carcinogenesis, 22:1399-1403). However, the manipulation of endogenous AR corepressors to downregulate AR function has not yet been reported. Androgen receptor antagonists used in the treatment of prostate cancer cause recruitment of corepressor complexes to the AR, which underlies their inhibitory activity. Shang et al. (Shang et al., 2002, Formation of the Androgen Receptor Transcription Complex, Mol. Cell, 9:601-610) have demonstrated that the androgen antagonist bicalutamide recruits the repressors NCoR and SMRT to the AR bound to the prostate specific antigen (PSA) promoter. Chen et al. (Chen et al., 2004, Molecular Determinants of Resistance to Antiandrogen Therapy, Nat. Med., 10:33-39) demonstrated that increases in AR protein levels lead to a decrease in corepressor recruitment to AR-regulated promoters after bicalutamide treatment. Conceivably, the lack of association of AR with inhibitory coregulators might contribute to the increased AR transactivation potency in prostate cancer. The clinical resistance of these agents may reflect a failure of corepressor recruitment to the AR.
Another potential approach toward prostate cancer is gene therapy. Intraprostatic injection has been proposed as a minimally invasive technique to deliver gene therapy that could be readily performed by urologists or radiologists. The unsolved issue is to identify genes that would be efficacious for therapy. The most extensively studied gene therapeutic approach to date for prostate cancer is suicide gene therapy, typically involving the tumor-targeted delivery of genes encoding metabolic enzymes that convert systemically delivered, relatively innocuous prodrugs into highly toxic metabolites. However, the effectiveness of these strategies for human prostate cancer may be blunted because of their limited effect on slowly dividing prostate cells that require long term administration of prodrugs to increase the proportion of cells affected. Furthermore, these genes can be inhibited by anti-apoptotic proteins like Bcl-2 family members, which have been shown to be upregulated in progressive, hormone-independent prostate cancer.
The prior art has thus far failed to provide successful methods of treatment for cancers such as prostate cancer, particularly for the hormone independent prostate cancer.